Hydroelectricity - Energy Policy Analysis: A Conceptual Framework - page 149

ment to effects of a hydropower facility, and steps taken to modify design and operation of each
facility to mitigate potential impacts (Power Scorecard 2011).
Construction of hydropower plants can alter large portions of land when dams are constructed and
lakes are created, flooding land that may have served as wildlife habitat, farmland, and recreational
areas (Armaroli and Balzani 2011, 234). Large reservoirs result in submersion of extensive areas
upstream of dams, destroying biologically rich and productive lowland and riverine valley forests,
marshland, and grasslands. Reservoirs cause wildlife habitat fragmentation of surrounding areas, and
may disrupt migration patterns of fish, mammals, and birds encountering unfamiliar terrain.
Stratification. Reservoirs can significantly slow the rate at which water moves downstream.
Surface temperatures tend to become warmer as the slower-moving or “slack” water absorbs heat
from the sun. Colder water sinks toward the bottom because of its higher density. This causes a
layering effect called stratification. The bottom layer is the coldest and the top layer the warmest,
and they tend not to mix. When stratification occurs, colder water that sinks toward the bottom
contains reduced oxygen levels. At some sites when water is released through a dam from the
colder, oxygen-depleted depths, downstream habitat conditions change because of reduced oxy-
gen in the water (FWEE 2011). Many fish that are adapted to stream conditions do not survive
in lakes. Often, water at the bottom of a lake created by a dam is inhospitable to fish because it
is much colder and oxygen-poor compared with water at the top. When this colder, oxygen-poor
water is released into the river, it can kill fish living downstream that are accustomed to warmer,
oxygen-rich water (USEPA 2011).
Changing Water Levels. Building a storage project can raise the water level behind a dam from
a few feet to several hundred feet. When stream banks and riparian areas become inundated by
a reservoir's higher water level, habitat conditions change permanently. As this occurs, a differ-
ent set of dynamics begin impacting species that traditionally grow, nest, feed, or spawn in these
areas. Once built, storage projects can also raise and lower the level of water in a reservoir on a
daily, weekly, or seasonal basis to produce electricity. In a riparian zone, where moist soils and
plants exist next to a body of water, this may result in shoreline vegetation not being effectively
reestablished (FWEE 2011). Some dams withhold water and then release it all at once, causing
the river downstream to flood suddenly. This action can disrupt plant and wildlife habitats and
affect drinking water supplies (USEPA 2011). Humans living on the shores of such a reservoir
may find it is not always pleasant, with vast expanses of recently inundated land exposed to the
air for months at a time, bearing the smell of decaying fish and other organic matter. And it may
not be visually attractive.
Generation of hydroelectric power changes the downstream river environment. Water exiting a
turbine usually contains little suspended sediment, lack of which can lead to scouring of downstream
riverbeds and loss of riverbanks. Since turbine gates are often opened intermittently, rapid or even
daily fluctuations in river flow are observed, with cyclic flow variation contributing to erosion
of downstream sandbars. Dissolved oxygen content of water may change from preconstruction
conditions. In some cases, an entire river may be diverted, leaving a dry riverbed.
Erosion. Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both up-
stream and downstream of a plant site. Changing water levels and a lack of streamside vegetation
can lead to increased erosion. Hydroelectric dams can cause erosion along the riverbed upstream
and downstream, which can disturb wildlife ecosystems and fish populations (USEPA 2011). Lack
of vegetation along a shoreline means that a river or reservoir can start cutting deeply into its banks.